1. Field of the Invention
The present invention relates to an infrared radiation detecting apparatus and a method for manufacturing it. More specifically, the present invention relates to a thermal type infrared radiation detecting apparatus employing an infrared radiation sensing device of a pyroelectric effect type and completed with a novel package structure and a method for manufacturing it.
2. Description of the Prior Art
A thermal type infrared radiation detecting apparatus those employing a pyroelectric effect type sensing device, a thermistor bolometer and the like have been proposed and put into practical use. FIG. 1 shows a sectional view of an infrared radiation detecting apparatus employing a prior art thermal type sensing device, which constitutes the background of the invention. The conventional infrared radiation detecting apparatus as shown in FIG. 1 comprises an insulating substrate 2, external connection pins 1 and 1' fixed to the insulating substrate 2 so as to extend through the insulating substrate 2, an infrared radiation sensing device 3 provided on the insulating substrate 2, and a hermetic base 5 fixed to the insulating substrate 2 through a spacer 4. A metallic casing 6 having an aperture 6a for receiving infrared radiation is hermetically sealed to the hermetic base 5 so as to enclose the insulating substrate 2. The aperture 6a of the metallic casing 6 is provided with a window member 6 of a material transmissible of infrared radiation of a desired wavelength region.
As well known, a package of such a hermetic seal structure performs an electric shield effect in view of the fact that a casing is metallic. In addition, such package provides a mechanically strong structure. Nevertheless, a thermal type infrared radiation detecting apparatus employing such package suffers from various shortcomings to be described in the following. Firstly, since such package structure is expensive, such package can be employed only in infrared radiation detecting apparatuses of particular applications which allow a high cost. Secondly, such package structure makes it difficult to implement a so-called longitudinal structure in which the external connection leads extend parallel to the substrate. Thirdly, such a package structure makes it difficult to employ flexible insulated lead wires as external connection leads. As the fourth disadvantage, since the casing is metallic, when the window member 7 of a resinous material such as polyethylene is adhered to the opening 6a by means of an adhesive agent, the adhesive agent comes into contact with the infrared radiation transmitting portion of the window member, which causes variability in the sensitivity of the detecting apparatus and degrades manufacturing efficiency. As the fifth disadvantage, since the heat capacity of the metallic casing is relatively large, dew is deposited on the surface of the casing when the ambient temperature of the detecting apparatus suddenly changes from low to high. Dew on the window member can interrupt the incident infrared radiation, and the moisture can enter the casing through the interface between the window member and the casing. As the sixth disadvantage, since the metallic casing is a good conductor of heat, and usually the substrate for supporting the sensing device is made of alumina which is also a good conductor of heat, any contact between the casing and the substrate accidentally occurring during assembly makes it difficult to detect slowly varying infrared radiation energy. As the seventh disadvantage, the metallic casing makes the package unavoidably bulky and difficult to miniaturize. As the eighth disadvantage, in the case where a field effect transistor is housed in the casing as a subsequent stage of the sensing device for the purpose of impedance conversion and amplification of the detected signal, it is required that the field effect transistor be coated with a resinous material in order to prevent the field effect transistor by being adversely affected from the moisture entering the casing, since commercially available field effect transistors resist moisture poorly.